No evidence for long‐term increases in biomass and stem density in the tropical rain forests of Australia

Abstract

Summary Pervasive increases in biomass and stem density of tropical forests have been recorded in recent decades, potentially having significant implications for carbon storage, biodiversity and ecosystem function. This trend is widely considered to be the result of multidecadal and global scale growth stimulation arising from increases in atmospheric CO2 and temperatures. However, contrasting patterns have been recorded across the tropics, and the role of disturbance in driving biomass and stem dynamics has been highlighted as an alternative explanation. Australian tropical forests have rarely been assessed in pan‐tropical analyses of long‐term dynamics. We have measured recruitment, mortality and growth in 20 permanent plots in tropical forest across north‐eastern Australia since 1971. We assess changes in plot level above‐ground live biomass (AGB) and stem density, and compare our results with those documented over a similar time frame in the neo‐tropics. No significant increase in AGB was found over the 40‐year time period. Above‐ground biomass tended to increase over the first two decades of the monitoring period and decrease in the final two with gain terms (growth and recruitment) lower than loss terms (mortality) by the final decade (2000s). Stem density significantly decreased over the monitoring period with recruitment consistently lower than mortality. There was large variation in individual plots in their pattern of AGB and stem density changes over time which was consistent with the response of each plot to known disturbance events, including cyclones, pathogen outbreaks and drought. Our results are in contrast to those described for neo‐tropical plots which appear to show a widespread pattern of increasing growth and stem density. Synthesis. The trend towards increasing biomass and stem density of tropical forests described for the neo‐tropics does not necessarily reflect patterns in areas of the tropics where large‐scale natural disturbances are relatively frequent. Australian tropical rain forests are either not increasing in productivity in response to global change, or cyclones and other regional and local mechanisms of change mask any evidence of larger‐scale patterns.